Using RAID (redundant array of inexpensive disks) improves fault tolerance and performance of disks. An exemplary RAID system 100 is shown in FIG. 1. A RAID array 102 consists of a plurality of disks 104, 106, 108, 110.
One example of a RAID system is a RAID4, which is a form of RAID where the data is striped across multiple data disks to improve performance, and an additional parity disk is used for error detection and recovery from a single disk failure. An example of RAID4 striping is shown in FIG. 2. A RAID 200 includes a plurality of data disks 202, 204, 206, 208, and a parity disk 210. The lettered portion of each disk 202-208 (e.g., A, B, C, D) is a “stripe.” To the user of the RAID 200, the RAID 200 appears as a single logical disk with the stripes laid out consecutively (A, B, C, etc.). A stripe can be any size, but generally is some small multiple of the disk's block size. In addition to the stripe size, a RAID4 system has a stripe width, which is another way of referring to the number of data disks, and a “slice size”, which is the product of the stripe size and the stripe width. A slice 220 consists of a data stripe at the same offset on each disk in the RAID and the associated parity stripe. Performance is improved because each disk only has to record a fraction (in this case, one fourth) of the data.
Removing and replacing a single disk from a RAID group triggers a reconstruction of the data that was on the removed disk. This data is reconstructed onto the replacement disk using the remaining data disks in the RAID group and the parity disk block by block. If a large disk is removed or a RAID group with a large number of disks has one disk removed, the reconstruction operation can be costly in terms of the time needed to reconstruct the removed disk and the processing resources used during the reconstruction operation. This problem is compounded in a storage system where multiple disks are located on a single carrier, such that all of the disks on the carrier are removed at the same time even if only one of the disks needs to be replaced.